1. Field of the Invention
The present invention relates to a holding apparatus and an optical apparatus.
2. Description of the Related Art
High positional accuracy is required for the relative positions of the constituent elements of an optical apparatus such as an astronomical telescope or a precision instrument such as an artificial satellite. Examples of the constituent elements of a precision instrument include a main body structure, an optical element, a sensor, and an actuator, and extremely high positional accuracy is required in particular for the optical element.
In general, the environmental temperature of a precision instrument differs greatly between the manufacturing/assembly environment and the usage environment. Specifically, in the case of an astronomical telescope, its manufacturing/assembly environment is often at a temperature in the neighborhood of 20° C., and its usage environment is often at a temperature in the neighborhood of 0° C. In the case of an artificial satellite, its usage environment is at the temperature in space, that is, at a temperature in the neighborhood of −270° C., and there also occurs a local temperature increase in the main body structure under irradiation with sunlight.
Such a temperature change leads to deformation of the constituent elements in accordance with their thermal expansion coefficients and accordingly causes a large misalignment in the relative positions of the main body structure and the optical element. Many constituent elements have a limited selection of materials in order to achieve their desired performance. For example, it is conceivable that a low thermal expansion material such as Invar may be used for a holding apparatus that holds an optical element, but in the case where glass constituting the optical element has a high thermal expansion coefficient, a titanium alloy or the like will be used in response thereto for the holding apparatus. In this case, misalignment due to a temperature change is inevitable and positioning repeatability cannot be ensured for each environment. In addition, distortion due to thermal stress occurs at coupling portions (joints) of the constituent elements. Note that such misalignment and distortion are factors that significantly degrade the performance of the optical element.
In view of this, in order to reduce distortion due mainly to temperature changes, technology for holding an optical element using kinematic mounts has been proposed in Japanese Patent Laid-Open No. 2004-247484 and by Varadarajan K. M. and Culpepper M. L. in “Active Compliant Fixtures for Nanomanufacturing”, Annual meeting of ASPE, 2004 (Document 1). Kinematic mounts are a holding method that constrains the degrees of freedom of a held rigid element as a rigid body just enough (that is, constrains the position of the rigid element in six degrees of freedom). With kinematic mounts, for example, one element that is provided with three spherical surfaces and another element that is provided with three V-shaped grooves are positioned by engagement of the spherical surfaces and the V-shaped grooves. Kinematic mounts have the property of not transmitting distortion and having excellent positioning repeatability.
However, since kinematic mounts uniquely determine the position of a target element, they may position the element at a position displaced considerably from the intended position in the case where the element has a large variation in temperature. For this reason, in the technology disclosed in Japanese Patent Laid-Open No. 2004-247484, a configuration has been proposed in which, in the case where deformation occurs due to a temperature change, a ball constituting a kinematic mount between a component targeted for holding and a structure is allowed to move via a movable groove component with a predetermined one degree of freedom. This enables the component targeted for holding to be held without being displaced in a plane even if deformation occurs due to a temperature change. However, with the technology disclosed in Japanese Patent Laid-Open No. 2004-247484, the positioning of the component targeted for holding cannot be performed along the reference axis.
Meanwhile, Document 1 discloses technology for arranging a component targeted for holding at an arbitrary position by controlling (changing) the angles of three V-shaped grooves that respectively hold three balls in the case where a shift in the reference position occurs due to deformation caused by a temperature change. However, since the technology disclosed in Document 1 necessitates a sensor for detecting the amount of shift of the reference position and an actuator for changing the angles of the V-shaped grooves, the configuration becomes complicated and increases in cost. Furthermore, power consumption and heat generation of the sensor and the actuator may have an adverse effect on the performance of a precision instrument and, in addition, high-precision positioning becomes difficult if deformation occurs in the basis for mounting the sensor and the actuator itself.